1. Field of the Invention
This invention relates to a new process for working up water-soluble cellulose derivatives having high flocculation points. For example, the process of the invention enables cellulose ethers of all kinds to be economically worked up.
2. Statement of Related Art
Methyl cellulose itself and derivatives thereof, for example methyl hydroxyalkyl celluloses, are used in a number of technical fields. Accordingly, their production on an industrial scale from inexpensively obtainable raw materials has recently received greater attention. In particular, processes by which methyl cellulose and its derivatives can be prepared in high yields under economically reasonable reaction conditions are acquiring increasing significance. In the context of such production processes which essentially comprise the steps of
(a) disintegrating the cellulose starting material (cotton linters, pulps of woods of various origin, etc.), PA1 (b) soaking the cellulose with alkalis, generally NaOH, to open up the lattice structure, PA1 (c) etherification (generally with alkyl halides and with alkylene oxides), PA1 (d) working up the crude product to remove unwanted secondary products and PA1 (e) conditioning,
the working up of the crude products (step (d)) is acquiring increasing significance. In this process step, the cellulose ether or cellulose mixed ether prepared in the etherification reaction is freed from the secondary products (methanol, dimethylether, (poly)alkylene glycols, NaCl, etc.) after discharge from the reactor. The product freed from sodium chloride and from the glycols with hot water is then delivered to the conditioning or working-up steps (drying, size-reduction, sieving, etc.).
To separate the by-product NaCl formed in quantities stoichiometric to the alkali metal hydroxide, the cellulose ether has hitherto normally been suspended in hot water, the sodium chloride dissolving immediately. In the prior art, the separation of the aqueous salt-containing phase from the cellulose ether is effected in centrifuges at temperatures of from 70.degree. C. to 90.degree. C.
To avoid losses of pure cellulose mixed ether in this process step, the suspension of the crude product in hot water and the subsequent separation by centrifuging have to be carried out distinctly above the flocculation point of the particular cellulose mixed ether, i.e. distinctly above the temperature at which the particuar cellulose ether flocculates from its solutions. At temperatures below the flocculation point, at least part of the crude product is dissolved and accordingly cannot be isolated in solid form.
Flocculation, which is characterized by the flocculation point, takes place over a relatively broad temperature range which is dependent upon the raw material used, upon the consistency of substitution, upon the degree of etherification and upon the product viscosity.
Although the requirement that suspension and centrifuging should be carried out above the flocculation point is readily satisfied for methyl cellulose itself because the flocculation point is between 35.degree. and 65.degree. C., depending on the average degree of substitution, problems arise for the hydroxyalkyl derivatives of methyl cellulose required in increasing quantities in industry due to the fact that their flocculation points increase distinctly even with low percentage contents of hydroxyalkyl groups in the polymer molecule. The same applies to the introduction of carboxymethyl groups into the methyl cellulose.
The working up of methyl cellulose derivatives by the abovedescribed and, hitherto, generally applied process became more difficult as the flocculation point rose increasingly through an increase in the degree of hydroxyalkylation for possibly lower methoxylation. In the case of methyl cellulose derivatives having flocculation points above 85.degree. C. and generally above 75.degree. C., clear product losses could not be avoided because the temperature in the suspension and in the separation step moved into the vicinity of or, in some cases, below the flocculation point. Cooling is intensified by air ventilation during the centrifuging step so that purified methyl cellulose derivative was partly dissolved. Hitherto, no measures have been proposed for changing this situation or rather for avoiding this problem.